The I region of the MHC encodes the Ia antigens or MHC class II genes. These proteins are necessary for recognition of foreign antigen on antigen presenting cells and on B cells by helper T cells in the periphery and they select the H-2 restricted, self-tolerant T cell repertoire in thymus. Based on our results and experience with animal model systems successfully generated as suggested in the previous proposal, the long term goal of the experiments outlined in this renewal application is to generate novel animal model systems to study the function of MHC class II genes in a normal immune response and to evaluate their contribution to an autoimmune response. In order to study MHC class II participation in normal immune responses, we will generate transgenic mouse line expressing the MHC class II I-E protein selectively on B cells. This will test in vivo the capacity of B cell I-E to prime T cells for proliferation to GLPhe and to generate an antibody response to this antigen. The combined approach of targeting E alpha gene expression in specific thymic tissues (already achieved) and in vivo cell ablation of MHC class II expressing tissues in the thymus will clarify the roles of specific class II-expressing thymic cell types in the selection of H-2 restricted T cells and in the induction of self- tolerance. A toxin and an inducible 2-component ablation strategy will be utilized in these studies. In order to assess the contribution of specific MHC class II alleles to autoimmune disease, we will generate a transgenic mouse line expressing the A beta (NOD) allele, a class II gene associated with IDDM, in a non- autoimmune genetic background. The internal A beta alleles will be inactivated by homologous recombination in embryonic stem cells and reconstitution of chimeric mice. We will test whether this A beta (NOD) allele contributes to the generation of an autoimmune attack on beta cells of the pancreas through failing to eliminate potentially self- reactive T cell clones during the thymic education process. A more detailed understanding of the function of MHC class II molecules, especially in the thymus where the selection and restriction of the T cell repertoire occurs, might allow future manipulation of or intervention in these processes, with the aim of ameliorating autoimmune disorders.